Justin Williams has filed documents in L.A. County Superior Court ... claiming he took his 10 month old son for a hair follicle drug test earlier this month after he began to suspect Kari Ann had fallen off the wagon and was using drugs around the kid.

Justin says the tests came back POSITIVE for meth.

In the docs, filed by his attorney Marlo Van Oorschot, Williams says he was initially concerned because the baby "has a habit of sucking on Kari Ann's hair and clothes and I became concerned that he could be affected by drug residue."

Justin also claims he found evidence that Kari -- who's struggled with addiction in the past -- had been using drugs in the past 2 months.

Now, Justin believes the baby is in danger when he is with Kari Ann ... explaining, "I do not want him to continue to be exposed to smoke from meth or to accidentally ingest illegal drugs while with Kari Ann."

Justin is demanding sole legal and physical custody -- and wants to BAN Kari Ann from any sort of unsupervised contact with the kid. He also wants Kari Ann to submit to weekly drug tests ... and if she tests positive, he wants the judge to revoke her visitation entirely.